At present, developments in high-power industrial microwave electrovacuum devices are aimed at maximizing the power output of individual devices by increasing their energy conversion efficiency. As is known, the power output of microwave devices and, more specifically, magnetron-type microwave devices, is restricted by the physical properties of the cathode, anode and dielectric energy exit window materials, and their capacity to withstand and dissipate electrical and thermal loads. These limitations are partially overcome by the following measures: the use of special materials with improved emission characteristics for cathodes, materials with high electric and heat conductivity for anodes and cathodes, materials with maximum permissible stability to thermal overloads and low dielectric loss for energy exit windows, etc. Besides, in order to enhance the power output in a single device, the heat-dissipating surface of the anode is enlarged by, for example, increasing the axial length of the cylindrical anode block, as well as by increasing the anode surface diameter.
Enlarging the surface of the anode block of microwave magnetron-type devices always involves an increase in the number of cells of the periodic structure of the retardation system. Therewith, the electrodynamic properties of a retardation system with an enlarged and adequately developed working surface of the anode and, in particular, the separation of frequency among competing modes are impaired, which imposes certain limitations on the possibility to minimize in this way the thermal and electrical loads on the anode and cathode, hence, on the possibility to further increase the microwave generating power in a single device.
Thus, all of the above measures fail to solve the problem of enhancing the energy conversion efficiency of microwave devices, i.e. minimizing undesirable losses, especially in continuously operating devices, and more particularly those intended for industrial use. This can be easily seen from the following formula for the maximum continuous-wave (or mean) power output of microwave magnetron-type devices: ##EQU2## where P is the maximum continuous-wave (or mean) microwave generation power output;
q is the maximum permissible specific load on the anode; PA1 S is the working anode surface; and PA1 .eta..sub.e is the electronic efficiency. PA1 B is the density of the applied working magnetic field; PA1 B.sub.0 is the minimum magnetic field density at which microwave generation is still possible in a microwave magnetron-type device. PA1 B.sub.0 is a constant component of the homogeneous magnetic field density along the anode block axis; PA1 B.sub.01 is the amplitude of variation in the magnetic field density along the anode block axis, over its length X.sub.1, which does not exceed 50% of B.sub.0 ; PA1 B.sub.02 is the amplitude of fluctuation of the magnetic field from one pair of retardation system straps to another along the anode block axis, over its length X.sub.1, which does not exceed 20% of B.sub.0 ; PA1 h is the spacing between strap pairs; PA1 n=1,2,3 . . . is a coefficient equal to the number of half-cycles of the cosinusoidal distribution of the amplitude to the high-frequency electric field of a respective mode.
The only feasible way to further increase the microwave generating power upon attaining the maximum possible values of q and S in microwave magnetron-type devices is by increasing the energy conversion efficiency, i.e. .eta..sub.e.
In a microwave magnetron-type device, the electronic efficiency .eta..sub.e depends to a great extent on the density of the applied magnetic field B which is at a right angle to the static electric field E applied between the anode and cathode of the device, i.e. ##EQU3##
where
However, the energy conversion efficiency in a microwave magnetron-type device depends not only on the density of the magnetic field B crossing the electric field E but also on the homogeneity and uniformity of the magnetic field B over the entire interaction space between the anode and cathode of the device. If these conditions are met and if the distribution of the magnetic field B corresponds to that of the total (static and high-frequency) electric field E in the interaction space, the energy conversion efficiency .eta..sub.e will be maximum and approach a value close to the theroretical one described in (2). Only in this case may the power output according to Eq. (1) be increased to the maximum possible value for devices of this type.
In microwave magnetron-type devices, the magnetic field along the working interaction space is created either with the aid of permanent magnets or by means of electromagnetic devices. However, the required homogeneity and distribution of the magnetic field over the entire interaction space cannot, as a general rule, be achieved solely by means of magnetic field sources, such as permanent magnets and electromagnets that are generally characterized by large scattering fields, which reduces the efficiency not only of the device but also of the magnets themselves, i.e. the ratio of the magnet mass to the actual (working) magnetic field density (M/B) is high. Therefore, in microwave magnetron-type devices, with a view to creating a homogeneous field in the working gap of the interaction space, obtaining a particular distribution pattern of the magnetic field along the interaction space, and enhancing the magnet efficiency, various improvements are introduced into the construction of the magnetic system, realizing the above objectives and, in the final analysis, ensuring stable operation of the device and attaining maximum possible efficiency.
The magnetic system of microwave magnetron-type devices normally comprises main and supplementary magnets in the form of a yoke, magnetic shunts, pole pieces, etc. which may have different embodiments and are arranged both along the periphery of the anode block in an external closed magnetic circuit, near the main magnets, and inside the evacuated housing of the device, in direct proximity to the interaction space of the device. The supplementary magnets are often made in the form of built-in pole pieces of a magnetic material (usually magnetically soft iron). Permanent magnets are most often used in microwave magnetron-type devices with a relatively short anode block and small interaction space--not more than a quarter wavelength (&lt;.lambda./4). In this case, the pole pieces in a microwave magnetron-type device with a cylindrical anode block are arranged between the permanent magnet poles, are connected thereto, and are shaped as cylindrical pieces, truncated cylinders, rings, etc. (cf. "Philips Electrical Ind. Ltd" Pat. No. 778,585 of Jan. 11, 1955; "Raytheon" Pat. No. 972,526 of Sept. 24, 1943; USSR Pat. No. 1,524,058 of June 21, 1966 ).
Electromagnets are used primarily in microwave magnetron-type devices with an axial length of the cylindrical anode block exceeding a quarter wavelength (&gt;.lambda./4) (cf. Journal of Microwave Power, 13 (1), 1978, pp. 59-64; C. Shibata, T. Akioka, V. Sato and H. Tamai, "100 kW, 915 MHz CW Magnetron for Industrial Heating Application"). In this case, in the magnetic system use is often made of magnetic shunts and pole pieces made of a magnetic material, which correct the shape and density distribution of the magnetic field created by the electromagnet along the axis of the anode block, which is particularly necessary if a solenoid is used to create a magnetic field in the interaction space of a device with a long anode block axis (exceeding .lambda./2). Annular magnetic shunts and pole pieces in such devices serve not only to decrease the magnetic field scattering and to improve its homogeneity in the interaction space (in order to provide for optimal conditions of interaction between the electron flow and the high-frequency field of the retardation system in the crossed magnetic field B and electric field E, thereby ensuring maximum possible energy conversion efficiency), but also to create in the interaction space, near the end faces of the anode block, magnetic traps preventing electrons from leaving the interaction space (for areas adjacent to the end faces of the anode block). Both measures minimize the undesirable losses (in the former case, due to interaction between electrons and the high-frequency field of the retardation system of the anode block and, in the latter, due to "leakage" of electrons into the areas near the end faces of the anode block) and increase the energy conversion efficiency .eta..sub.e.
In microwave magnetron-type devices with great axial lengths of the interaction space, in which long anode blocks are most frequently used and are formed by multistage retardation system structures, the amplitude of the high-frequency electric field along the axis of the anode block varies cosinusoidally. This is why, in contrast to devices with a short anode block, here the conditions for interaction between the electron flow and the high-frequency field in the interaction space more closely adjacent to the end faces of the anode block are substantially different from those for interaction in a space which is closer to the mid-portion of the anode block. As a result, the energy conversion efficiency in a microwave magnetron-type device with a long anode block is always lower than in devices of the same type with a short anode block.
Thus, in devices with a long anode block, there is a problem of enhancing the efficiency of interaction between the electron flow and the high-frequency field of the retardation system with due account taken for the high-frequency component of the electric field (E), which problem is solved by creating, in the crossed fields E and B, identical conditions for interaction over the entire length of the anode-cathode spacing. For example, such conditions are provided by changing, in a certain manner, the diameters of the anode and cathode along the anode block axis (i.e. by varying the constant electric field E along the anode block axis, between the anode and cathode), as well as by designing the solenoid and the entire magnetic system of the device so as to provide for the required configuration and density distribution of the magnetic field B along the axis of the anode block.
Known in the art is a microwave magnetron-type device comprising a long cylindrical anode block with a multicavity retardation system without straps, in which the length of the interaction space along the anode block axis, confined by the gap between the cylindrical anode opening and the cylindrical cathode, is extended to a value approximately equal to a wavelength of the generated waves (cf. Booth, "Magnetrons with Long Anode", "Electronic Microwave Devices with Crossed Fields", vol. 2, Moscow, 1961, pp. 236-248/in Russian).
The magnetic field in the interaction space along the axis of the long anode block in the prior art device is created by means of a magnetic system including a solenoid and pole pieces made of a magnetic material, the pole pieces being secured on the edges of the solenoid orifice in direct proximity to the end faces of the anode block. To provide for a more homogeneous magnetic field in the working part of the interaction space, as well as to increase the magnetic field density and to change its direction near the end faces of the anode block in order to create magnetic traps preventing electrons from being ejected into the areas near the anode block end faces, bushings of various configurations, made of a material with high permeability, are provided (cf. "Marconi" Pat. No. 523,329 of Dec. 30, 1937; Journal of Microwave Power, 10 (2), 1975 "High Power Industrial Heating Magnetron development", C. B. Bighom and M. Viant).
In this microwave device with a long anode, the axial distribution of the amplitude of the high-frequency electric field in the interaction space is markedly dependent on the length of the anode block, and in the mid portion of the anode block the amplitude may be several times greater than at the ends. This results in the fact that the conditions of interaction between the electron flow and the high-frequency field of the operating mode, in the case of a homogeneous axial magnetic field created by the magnetic system of the solenoid, are dissimilar in different parts of the interaction space of the device. As a consequence, since there is a correspondence between any arbitrarily selected part of a long interaction space and its optimal performance with respect to the anodic current and anodic voltage, the region of the current-voltage characteristic of the device, in which the electronic efficiency reaches maximum values, is not pronounced; hence, the resulting efficiency is averaged, and is lower than normal for microwave magnetron-type devices.
These are the reasons why the microwave power output in the above device cannot be substantially increased, particularly in continuous-wave generation, by further extending the length of the anode block because the operation of the device becomes less stable and, in the extreme case, the device becomes inoperable (already at an anode block length exceeding a wavelength of the generated waves).
Also known in the art is a microwave magnetron-type device comprising at least one anode block with annular metal straps electrically associated with respective vanes of cavities in each anode block, having the same polarity at .pi.-mode, and straps of different polarities, electrically associating respective vanes of the same polarity and paired in each anode block, each pair being arranged with respect to one another along the axis of a respective anode block so as to form a single multistage retardation system, means for creating a magnetic field directed along the axis of each anode block, embracing all anode blocks, and means for increasing the magnetic field density in direct proximity to the end faces of a respective anode block, forming a gap with the end faces of each anode block (cf. U.S. Pat. No. 3,045,147; Nov. 16, 1959; Cl. 315-39.69).
The means for increasing the magnetic field density, arranged in direct proximity to the end faces of the anode block and made in the form of pole pieces or rings to additionally improve the homogeneity of the magnetic field in the working part of the interaction space and to enhance the magnetic field density in direct proximity near the end faces, forms together with the magnetic field creating means, e.g. a solenoid or electromagnet, the magnetic system of the device.
In this microwave magnetron-type device with a long anode block, the straps periodically arranged along the retardation system of the anode block are made of a nonmagnetic material. The pole pieces are made of a highly permeable material to eliminate the end-face effects, i.e. to minimize ejection of electrons into the areas adjacent to the end faces of the anode block from the interaction space, and to improve the interaction between the electron flow and the high-frequency field of the retardation system in the rest of the working part of the interaction space.
Since, in the above microwave magnetron-type device with a long anode block comprising a multistage retardation system, the distribution pattern of the amplitude of the high-frequency electric field in the interaction space along the anode block axis is not taken into account in the corresponding optimal axial distribution pattern of the magnetic field density, undesirable physical phenomena occur in the operating device, leading to a poorer efficiency and stability thereof. For example, a variation in the amplitude of the high-frequency electric field between the straps of the retardation system and a variation in the total amplitude of the high-frequency electric field over the entire length of the interaction space result in that (in the case of a homogeneous magnetic field) the power of the electron back bombardment of the cathode arranged coaxially with the anode block is dissimilar in different portions of its surface along the axis. Thus, electrical and thermal loads on the cathode surface are unevenly distributed, which leads to such undesirable consequences as a poorer efficiency of the device, premature wear of the current-overloaded portions of the cathode, hence, shorter life of the device, changes in the cathode geometry and its orientation with respect to the anode block axis, poorer stability of the device in operation, etc. After operation, localized wear spots are observed on the emitting surface of the cathode, opposite the retardation system portions with maximum high-frequency amplitude in the interaction space (under straps and under the central portion of the anode block).
Closer to the center of the interaction space, considerable Coulomb forces also occur due to the nonuniform density of the space charge, which forces cause instability in the interaction of electrons with the high-frequency electric field of the retardation system, as well as leakage of electrons towards the anode (so-called dark currents which are particularly dangerous in continuous-wave generation).
As a result, the possibilities of improving the efficiency of microwave magnetron-type devices with a long anode cannot be adequately utilized. Usually, the efficiency of such devices is relatively low, amounting to 50-60%, and in continuous-wave generation they may fail, which limits the area of their application.